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Texas Guide to Rainwater Harvesting

TEXAS GUIDE TO

Texas Water Development Boardin Cooperation with the

Center for Maximum Potential Building Systems

1997 AUSTIN, TEXA S

Second Edition


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SECOND EDITION ACKNOWLEDGMENTS

This is the second edition of this publication. In this edition, the staff of the TexasWater Development Board have ad ded several significant new pieces of information

and have modified others as more information becomes available in this rap idly

changing field. This has resulted in a text which differs somewhat from the original text

written by the Center for Maximum Potential Building Systems. As new information

becomes available, the Board staff will make ap propriate changes in future ad ditions as

time and funds permit. In add ition to the recognition of contributors presented in the

original acknowledgment below, the Texas Water Development Board staff would like

to recognize Matthew Bachardy, Harley and Pam Rose, Peter Pfeiffer, Kate Houser,

Duncan Echelson, Jeff Reich, and others who helped contribute to the second editions

technical content.

ORIGINAL ACKNOWLEDGMENTS

The Texas Guide to Rainwater Harvesting was developed by the Center for Maximum

Potential Building Systems, Austin, Texas, und er the direction of Gail Vittori, with

funds p rovided by the Texas Water Development Board Contract #95-483-136. The

Guide was researched and w ritten by Wendy Price Todd, AIA and Gail Vittori, and

edited by Bill Hoffman , P.E., Ken Heroy and Janie Hop kins. Graph ic design and

prod uction was prov ided by Worldwise Design Creative Director Harr ison Saunders.

Spanish translation by Cr istina Villarreal.

The Center for Maximu m Poten tial Building Systems, established in 1975 and based

in Austin, Texas, is a non-profit education, research, and demonstration organization

ded icated to sustainable planning, design and development. For more information on

the Center s activities, contact u s at 8604 F.M. 969, Austin, TX 78724, 512-928-4786.

The authors would like to thank the following agencies and ind ividuals for their

valuable assistance: Laurence Doxsey, City of Austin Green Builder Program ; Marcia

Roberts, Lower Colorado River Auth ority; Mike McElveen; Mary Sanger, Texas Center

for Policy Stud ies; Hu go Gard ea, Texas Historical Commission; Ron Bearden and

Wendi White, Texas Natu ral Resource Conservation Comm ission; Dennis J. Lye, U.S.

Environm ental Protection Agency; Yu-Si Fok, University of Hawaii; Phillip S. McClay,

Water Filtration Comp any; Joe Henderson, Water Qu ality Association; the contractors

listed in the Resource section; and the participants in the Case Studies.


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IS RAINWATER HARVESTING FOR YOU? .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .iINTRODUCTION ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

I . THE WATER CYCLE... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

II. ADVANTAGES OF RAINWATER ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Environ mental Ad vantages ................................................................. 2Qu alitative Advantages ....................................................................... 2

III. WATER QUALITY CONSIDERATIONS.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..3Primary Water Quality Cr iteria H ealth Concerns ........................ 3Secondary Water Quality Cr iteria Aesthetic Concerns ................ 4

IV. HOW DOES A RAINWATER HARVESTING SYSTEM WORK?. . . . . . . . . . . . 6 System Com ponents ............................................................................. 6 A. Catchm ent Area .............................................................................. 6

B. Gutters and Downspouts(with Screens and Roofwashers) ............. 7 C. Storage Tanks .................................................................................. 9D. Con vey ing ....................................................................................... 12E. Water Treatment(Filters andDisinfection) .................................... 13

V. HOW MUCH WATER DO YOU USE? ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Household Water Budget .................................................................... 17 Landscape Water Budget ..................................................................... 18Water Conservation Techn iqu es ......................................................... 18

VI. HOW MUCH WATER CAN YOU COLLECT? .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .22Rainfall Data for Selected Communit ies Across Texas ................... 27

VII. COST CONSIDERATIONS.. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

VIII.CODE AND SAFETY ISSUES ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

IX. CASE STUDIES .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Masonry and Concrete ........................................................................... 36 Masonry .................................................................................................. 36 Cast in Place Concrete ............................................................................ 37 Gu nnite ................................................................................................... 38Ferrocement ............................................................................................ 38Plastic .................................................................................................... 42Fiberglass ................................................................................................ 42Polyethylene ........................................................................................... 47 Metal .................................................................................................... 48

Steel .................................................................................................... 48Composite Systems ................................................................................ 51Composite ............................................................................................... 51In Process ................................................................................................ 53

XI. APPENDIX... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Glossary .................................................................................................. 54Abbreviation s ........................................................................................ 55References .............................................................................................. 55

CONTENTS


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he Texas Guide to Rainwater Harvestingis a primer of the basic principles of captured rainfall, with an emphasis onresidential and small-scale commercial

applications. If you are considering rainwaterharvesting as a partial or total source of yourwater supply for new construction or remodel-ing, this Guide and accomp anying videotape pro-vide the essential information to enable you todesign a system that meets your needs.

Most Texans have not had to operate their ownwater system. Your utility has done that for you .If you p lan to use a rainw ater harvesting systemfor your source of drinking water and for otherdirect human purposes, you must be willing tomake a commitment to its long term, properoperation and maintenance, or you could endan-

ger your familys and friends health. Your localhealth department and city building code of-ficer should also be consulted concerning safe,sanitary operations and construction of thesesystems.

As you read this manual, seriously considerwhat you want your system to do and how youwill provide back-up water if you are designingthe system as a sup plemental water source, or inthe event of severe drought. The case studies,covering several dozen installations operating in

Texas, provide an excellent snapshot of currentsystems.

What makes rainwater harvesting the pre-ferred water source for some Texans today?While large, sophisticated systems are notcheap, some Texans have devised innovativeapproaches that are both effective and afford-able. Rainwater catchment systems provide asource of soft, high quality water, reduce reli-ance on wells and oth er water sources, and , inmany contexts, are cost-effective. Systems canrange in size from a simple rain barrel to acontractor designed and built system costingthousand s of dollars. How ever, rainwater har-vesting systems are inherently simple in form,and can often be assembled w ith readily avail-able materials by owner-builders with a basic

understanding of plumbing and constructionskills. If you p lan to use the water for hum anconsumption, it is wise to consult or employexpert s. Texans with the time an d the inclina-tion to build their own system can save asignificant portion of costs associated with la-bor. Regardless of wh ether you in tend to hire acontractor or build a system yourself, we rec-ommend that you read through the entiremanual before starting a catchment system of your own.

IS RAINWATER HARVESTING FOR YOU?

T

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or centuries in Texas and throughout theworld, people have relied on rainwater har-vesting to supply water for household, land-scape, livestock, and agricultural uses. Be-

fore large, centralized water supply systems weredeveloped, rainwater was collected from a varietyof surfacesmost commonly roofsand stored onsite in tanks known as cisterns. With the advent of large, reliable community treatment and distribu-tion systems and more affordable well drillingequipment, rain harvesting systems have been allbut forgotten, even though they offer a source of pu re, soft, low sodium water. A renewed interest inthis time-honored approach has emerged in Texasand elsewhere due to:

s the escalating environm ental and economic costsof providing water by centralized water systems or

by well drilling;s health concerns regarding the source and treat-ment of polluted waters;

s a perception that there are cost efficiencies associ-ated w ith reliance on rainwater.

From rock cisterns to hollowed out tree trunks,historical precedents abound that trace peoplesreliance on rainwater collection. The Hueco Tanks

in west Texas are natural rock basins that trappedrainwater for the native dwellers, from the archaichunters to the Mescalero Apaches, and later be-came a stopping point for stagecoach travelers. Insouth Texas and the Rio Grande Valley, centralplazas were often not only the place where thetownspeople congregated for social affairs, butalso were the collection surfaces for vast under-

ground tanks that collected and stored water for useby adjacent shops and homes. Such notable historicstructures as the Stillman House in Brownsville, theFulton Mansion near Rockport, the Freeman Planta-tion near Palestine and the Carrington-CouvertHouse in Austin collected rain from their roofs, andthen guttered and piped the water into an above-ground tank or cellar cistern. While many of thesesystems are no longer in use, they signify the impor-tance that early Texas settlers placed on capturedrainfall for sustenance.

Today, island states such as Hawaii and entire conti-nents such as Australia promote rainwaterharvesting as the principal means of supplying house-hold water. In Bermuda, the U.S. Virgin Islands andother Caribbean islands where rainwater is the mostviable water supply option, public buildings, private

houses, and resorts collect and store rainwater. And inHong Kong, skyscrapers collect and store rainwater tosupply the buildings water requirements.

As with other natural systems, rainfall maintains itsown cycles and patterns as evidenced by the severedroughts that devastated the Texas landscape in the1950s, as well as the floods of 1981 and 1993 thatravaged east, south, and central Texas. These extremesunderscore the importance of designing your rainwatercatchment system with a thorough understanding of

the basic principles and essential information containedin this Guide.

As you will see in the following pages, many Texanstoday are putting their dollars behind a life-long invest-ment in a rainwater harvesting system over otheroptions. A decision to reduce household water con-sumption to live within your means is a commitmentthat may not be for everyone, but it may be for you.

ii

FINTRODUCTION


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THE CARRINGTON-COUVERT HOUSE(1857)Colorado Street, Austin, Texas

Colorado Street

16thStreet

Cistern

ROOF PLAN

North

EAST ELEVATION

WEST ELEVATION

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he never-ending exchange of water fromthe atmosphere to the oceans and backagain is known as the hydrologic cycle.This cycle is the source of all forms of

precipitation (hail, rain, sleet, and snow), and thusof all water. Precipitation stored in streams, lakes,and soil evaporates while water stored in plants

transpires to form clouds which store the water inthe atmosphere.

Currently, about 75% to 80% of conventionalwater supplies from lakes, rivers, and wells aredeveloped and in use in Texas. Making the mostefficient use of our States limited and preciousresources is essential. This includes using appli-ances and plumbing fixtures that conserve water,not wasting water, and taking advantage of alter-

native water sources such as greywater reuse andrainwater harvesting.

PERMEABLE ROCKSCONTAINING FRESHGROUNDWATER(WATER TABLE)

ROCKS PERMEABLEBUT NOT SATURATED

IMPERMEABLE ROCK S

SALTWATER INTERFACE

RECHARGE

RUNOFF INSTREAMS

EVAPORATIONAND TRANSPIRATIO N

BY PLANT S

EVAPORATIO N FROM OCEAN

W ATER VAPOR TRAN SPORT, OC EANTO OCEAN AND OCEAN TO LAND

OCEAN

SALINE GROUNDWATER

w

w

w

w

TI. THE WATER CYCLE

w

w

w

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or some Texans, rainwaters environmentaladvantages and purity over other water op-tions make it their top choice, even with their knowledge that precipitation cycles can fluc-

tuate from year to year.

ENVIRONMENTAL ADVANTAGESCollecting the rain that falls on a bu ilding to be usednearby is a simple concept. Since the rain youharvest is independent of any centralized system,you are promoting self-sufficiency and helping tofoster an ap preciation for this essential and p reciousresource. Collecting rainwater is not only waterconserving, it is also energy conserving since theenergy input required to operate a centralized watersystem designed to treat and pump water over avast service area is bypassed. Rainwater harvesting

also lessens local erosion and flooding caused byrunoff from impervious cover such as pavementand roofs, as some rain is instead captured andstored. Thus, stormwater run-off, the normal conse-quence of rainfall which p icks up contaminants anddegrad es our waterways, becomes captured rainfallwhich can then fulfill a number of prod uctive uses.Policymakers may wish to reconsider present as-sumptions regarding impervious cover and conse-quent run-off management strategies when rainwa-

ter harvesting systems are installed.

QUALITATIVE ADVANTAGESA compelling advantage of rainwater over otherwater sources is that it is one of the purest sources of

water available. Indeed, the quality of rainwater isan overriding incentive for people to chooserainwater as their primary water source, or forspecific uses such as watering houseplants andgardens. Rainwater quality almost always exceedsthat of ground or surface waters: it does not comeinto contact with soil and rocks where it dissolves

salts and minerals, and it is not subject to many of the pollutants that often are discharged into surfacewaters such as rivers, and which can contaminategroundwater. However, rainwater quality can beinfluenced by where it falls, since localized indus-trial emissions affect its purity. Thus, rainwaterfalling in non-industrialized areas can be superior tothat in cities dominated by heavy industry, or inagricultural regions where crop dusting is preva-lent.

Rainwater is soft and can significantly reduce thequantity of detergents and soaps needed for clean-ing, as compared to typical municipal tap water.Additionally, soap scum and hardness depositsdisappear, and the need for a water softener, oftenan expensive requirement for well water systems, iseliminated. Water heaters and pipes will be free of deposits caused by hard water and should lastlonger. Rainwaters purity also makes it an attrac-tive water source for certain industries for whichpure water is a requirement. Thus, industries suchas computer microchip manufacturing and photo-graphic processing may also wish to examine thissource of water.

Adv antages of Rainwater

FII. ADVANTAGES OF RAINWATER


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eople who relied on rainwater systems 30 to40 years ago may well recall contaminationas a serious concern. Because the construc-tion methods and materials used to build

many of the rural cisterns were not in compliance withtodays standards, and because of inadequate treat-ment procedures, illnesses associated with drinkingunhealthful water were not uncommon. However,rainwater can provide clean, safe, and reliable water solong as the collection systems are properly built and maintained, and the water is treated appropriately for intended uses.

PRIMARY WATER QUALITY CRITERIA HEALTH CONCERNSOnce rain comes in contact with a roof or collectionsurface, it can w ash many types of bacteria, molds,

algae, protozoa and other contaminants into the

cistern or storage tank. Indeed, some samples of harvested rainwater have shown detectable levelsof these contaminants. Health concerns related tobacteria, such as salmonella, e-coli and legionella,and to physical contaminants, such as pesticides,lead, and arsenic, are the primary criteria for drink-ing water quality analysis. Falling rain is free of

most of these hazards. Common sense takes a lot of the guess work out of proper treatment procedures.

For examp le, if the rainwater is intended for useinside the household, either for potable uses suchas drinking and cooking or for non-potable usesincluding showering and toilet flushing, appropri-ate filtration and disinfection practices should beemployed. If the rainwater is to be usedoutside forlandscape irrigation, where human consumption

of the un treated water is less likely, the presence of

More than 1,000 tons per year

500 to 1, 000 tons per year

100 to 500 tons per year


Less than 10 tons per year

ANNUAL SULFUR DIOXIDE (SO2) EMISSIONS IN TEXASBY COUNTY, 1994

Source:- Texas Natural ResourceConservation Commission

ANNUAL PARTICULATE MATTER (PM10) EMISSIONS INTEXAS BY COUNTY, 1994

More than 1,000 tons per year

500 to 1, 000 tons per year



Less than 10 tons per year

PIII. WATER QUALITY CONSIDERATIONS


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contaminants may not be of major concern and thustreatment requirements can be less stringent or notrequired at all.

Depending on where the system is located, the

quality of rainwater itself can vary, reflecting expo-sure to air pollution caused by industries such ascement kilns, gravel quarries, crop dusting, and ahigh concentration of automobile emissions.

SECONDARY WATER QUALITY CRITERIA AESTHETIC CONCERNSAesthetic concerns such as color, taste, smell, andhardness comprise the secondary testing criteriaused to evaluate publicly supplied water. When

assessed according to these characteristics, rain-water proves to be of better quality than well ormunicipal tap water. Inorganic impurities such assuspended p articles of sand, clay, and silt contributeto the waters color, and smell. Proper screening andremoval of sedimentation help to decrease problemscaused by these impurities.

Rainwater is the softest natural occurring wateravailable, with a hardness of zero for all practicalpurposes. In central and west Texas, dust derivedfrom limestone and alkaline soils can add as muchas one or two milligrams per liter (mg/ L) of hard-

ness to the water, although these amounts are negli-gible compared to the average hardness (about 200to 400 mg/ L) of groundwater in som e areas. Asmentioned above, a benefit of the soft water is that

faucets and water heaters last longer without thebuild-up of mineral deposits.Rainwater contains almost no d issolved minerals

and salts and is near distilled water quality. Totaldissolved minerals and salts levels average about10 milligrams per liter (mg/ L) across Texas. TotalDissolved Solids (TDS) can range as high as 50mg/ L and as low as 2.0 mg/ L. These values arevery low when compared to city tap water acrossTexas, which typ ically is in the 200 to 600 mg/ L

range, making rainwater virtually sodium free. Forpeople on restricted salt diets, this represents adecisive advantage over other water sources.

The pH of rainfall would be 7.0 if there werenothing else in the air. However, as rain fallsthrough the air, it dissolves carbon dioxide that isnaturally present in the air and becomes slightlyacidic. The resultant pH is 5.6; how ever, any su lfatesor nitrates dissolved from the air will lower thisnumber below pH 5.6. According to National Atmo-spheric Deposition Program d ata, the pH of rainfallin Texas ranges from 4.6 in east Texas to 5.6 or above

Water Quality Considerations

WATER QUALITY PROPERTIES RELATED TO SPECIFIC USESDOMESTIC INDUSTRIAL IRRIGATION

Taste pH BoronOdor Acid ity Alkalinity

Poisons Alkalinity Sodium-Calcium RatioFlouride Silica Dissolved solids

Nitrate HardnessIron SedimentHardness Dissolved solidsSediment

Dissolved solids


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in west Texas. While northeast Texas experiences aneven lower pH than found in other parts of the state,acid rain is still not considered a serious concernthroughout Texas.

Although the pH of rain is below neutral, it isonly slightly acidic, and the smallest amount of buffering can neutralize the acid. The low totaldissolved salts and minerals levels found in rainw a-ter permit even very small amounts of somethinglike baking soda (one level tablespoon per 100gallons) to adjust the pH to near neutral.

The Texas Natural Resource ConservationCommission (TNRCC) monitors municipal water

quality and has adopted Drinking Water Standardsin accordance with the Federal Safe Drinking WaterAct. If you plan to use your harvested rainfall fordrinking water, have the water tested by a labora-

tory certified by the Texas Department of Health(TDH) or Environmental Protection Agency(EPA). A list of drinking water testing criteria canbe obtained from TNRCC or TDH. The Texas De-partm ent of Health p erforms tests for coliform bac-teria for a nominal fee at locations around the state.At least 100 ml. of water are required to perform thetest; results are available within five days.

PH SCALE FROM BASIC TO ACID pH is the measure of acidity or alkalinity. In a scale from 0 to 14, 7 is neutral, values less than 7 represent more acid conditions, values greater than 7 represent more basic or alkaline conditions. The determination of whether water isacidic, neutral, or basic, is referred to as pH, which is a measure of the hydrogen ion concentration in water. The desired pH of potable water is pH 7, while the scale ranges from values of less than pH 7 down to pH 1 as increasingly acidicand greater than pH 7 up to pH 14 as increasingly basic. Soda pop and vinegar have a pH of about 3.0.

Bleach

Ammonia

Milk ofMagnesia

Borax

Baking Soda

Sea Water

Blood

Distilled Water

M i l k

Corn

Rainwater

Orange Juice

Vinegar

Lemon Juice

Battery Acid


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SYSTEM COMPONENTSWhether the system you are planning is large orsmall, all rainwater harvesting systems are com-prised of six basic components:A. Catchment Area/Roof, the surface upon which

the rain falls;B. Gutters and Downspouts, the transport chan-

nels from catchment su rface to storage;C. Leaf Screens and Roofwashers, the systems

that remove contaminants and debris;D. Cisterns or Storage Tanks, where collected rain-

water is stored;E. Conveying, the delivery system for the treated

rainwater, either by gravity or pump; andF. Water Treatment, filters and equipment, and

additives to settle, filter, and disinfect.

Storage Tank

Conveying andWater Treatment

Gutters andDownspouts

Catchment Area/Roof

RAINWATER HARVESTING SYSTEM M AIN COMPONENTS

A. CATCHMENT AREAThe catchment area is the surface on which the rainthat will be collected falls. While this Gu ide focuseson roofs as catchment areas, channeled gulliesalong driveways or swales in yards can also serveas catchment areas, collecting and then directing

the rain to a french d rain or bermed detention area.Rainwater harvested from catchment surfacesalong the ground, because of the increased risk of contamination, should only be used for lawn wa-tering. For in-home use, the roofs of buildings arethe primary catchment areas, which, in rural set-tings, can include outbuildings such as barns andsheds. A rainbarn is a term describing an open-sided shed designed with a large roof area forcatchment, with the cisterns placed inside alongwith other farm implements.

IV. HOW DOES A RAINWATER HARVESTINGSYSTEM WORK?

How Does a Rainwater Harvesting Sy stem W ork?


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b

a

ba

ROOF

A R E A

CALCULATING CATCHMENT AREA

B. GUTTERS AND DOWNSPOUTSThese are the components w hich catch the rain fromthe roof catchment surface and transport it to thecistern. Standard shapes and sizes are easily ob-tained and maintained, although custom fabricatedprofiles are also available to maximize the total

amount of harvested rainfall. Gutters and down-spou ts must be properly sized, sloped, and installedin order to maximize the quantity of harvested rain.

MATERIALS AND SIZES.

The most common material for off-the-shelf guttersis seamless aluminum, with standard extrusions of 5 inch and 6 inch sections, in 50 foot lengths. A 3inch downspou t is used w ith a 5 inch gutter and a 4inch downspout is used w ith a 6 inch gutter. Galva-nized steel is another common material which canbe bent to sections larger than 6 inches, in lengths of 10 feet and 20 feet. A seamless extrud ed aluminum6 inch gutter with a 4 inch downspout can hand leabout 1,000 square feet of roof area and is recom-mended for most cistern installations. For roof areasthat exceed 1,000 square feet, larger sections of gutters and downspouts are commonly fabricated

Rainwater yield varies with the size and textureof the catchment area. A smoother, cleaner, andmore impervious roofing material contributes tobetter water quality and greater quantity. Whileloss is negligible for pitched metal roofs,concrete or asphalt roofs average just less than10% loss, and built up tar and gravel roofs averagea maximum of 15% loss. Losses can also occur inthe gutters and in storage. Regardless of roofingmaterial, many designers assume up to a 25% losson annual rainfall. These losses are due to severalfactors: the roofing material texture which slowsdown the flow; evaporation; and inefficiencies inthe collection process.

WHAT TYPE OF ROOFING MATERIAL?If you are planning a new construction project,metal roofing is the preferred material because of itssmooth surface and durability. Other materialoptions such as clay tile or slate are also appropriatefor rainwater intended to be used as potable water.These surfaces can be treated with a special paintedcoating to discourage bacterial growth on an other-wise porous surface. Because composite asphalt,asbestos, chemically treated wood shingles and

some painted roofs could leach toxic materials intothe rainwater as it touches the roof surface, they arerecommended only for non-potable water uses.

For systems intended as potable water sources, no lead is to be used as roof flashing or asgutter solder as the slightly acid quality of rain candissolve the lead and thereby contaminate water supply. Existing houses and buildings should be fully examined for any lead content in the planning

stages of any rainwater collection project.

CATCHMENT AREA SIZEThe size of a roof catchment area is the buildingsfootprint under the roof. The catchment surface islimited to the area of roof which is guttered. Tocalculate the size of your catchment area, multi-ply the length times the width of the gutteredarea (See Chapter VI for m ore d etail).



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How Does a Rainwater Harvesting System W ork?

EXAMPLE OF A COMMERCIALLY AVAILABLEROOF WASHER WITH FILTER SYSTEM

from galvanized steel or the roof area is divided intoseveral guttered zones. Downspouts are designed tohandle 1.25 inches of rainfall during a 10 minuteperiod.

Copper and stainless steel are also used for guttersand downspouts but at far greater expense than eitheraluminum or galvanized steel. Downspouts are typi-

cally the same material as the gutters but of a smallercross section. The connection between the downspoutto the cistern is generally constructed of Schedule 40PVC pipe.

To keep leaves and other debris from entering thesystem, the gutters should have a continuous leaf screen, made of 1/ 4 inch wire mesh in a metal frame,installed along their entire length, and a screen or wirebasket at the head of the downspout. Gutter hangers

are generally placed every 3 feet. The outside face of the gutter should be lower than the inside face toencourage drainage away from the building wall.Where possible, the gutters should be placed about1/ 4 inch below the slope line so that debris can clearwithout knocking down the gutter.

As with the catchment surface, it is important to ensure that these conduits are free of lead and anyother treatment which could contaminate the water.Check especially if you are retrofitting onto older guttersand downspouts that may have lead solder or lead-based

paint.ROOF WASHERS

Roof washing, or the collection and disposal of thefirst flush of water from a roof, is of particular

concern if the collected rainwater is to be used forhuman consumption, since the first flush picks upmost of the dirt, debris, and contaminants, such asbird droppings that have collected on the roof andin the gutters du ring dry periods. The most simpleof these systems consists of a stand pipe and agutter downspout located ahead of the downspoutfrom the gutter to the cistern. The pipe is usually 6or 8 inch PVC which has a valve and clean out atthe bottom. Most of these types of roofwashersextend from the gutter to the ground where theyare supp orted. The gutter downspout and top of the pipe are fitted and sealed so w ater will not flowout of the top. Once the pipe has filled, the rest of the water flows to the d ownspout connected to thecistern. These systems should be designed so thatat least 10 gallons of water are diverted for every1000 square feet of collection area. Rather thanwasting the water, the first flush can be used for non-

potable uses such as for lawn or garden irrigation.Several types of commercial roof washers whichalso contain filter or strainer boxes are available.

Consider trimming any tree branches that overhang the roof. These branches are perches for birdsand produce leaves and other debris.

Leaf Screen

Basket Strainer

Gutter Outlet

Screen

To Cistern

Clean out & Valve

Roof Washer

Gutter

Courtesy of W ater Filtration Company

EXAMPLE OF A STANDPIPE TYPE ROOF WASHER


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CoverInlet

Water Level

Sediment

A TYPICAL STORAGE CISTERN

Outflow

SightingGauge

Overflow

C. STORAGE TANKSOther than the roof, which is an assumed cost in mostbuilding projects, the storage tank represents the larg-est investment in a rainwater harvesting system. Tomaximize the efficiency of your system, your buildingplan should reflect decisions about optimal placement,capacity, and material selection for the cistern.

SITING

In Texas, recently installed cisterns are placed bothabove and below ground. While above ground instal-lations avoid the costs associated with excavation andcertain maintenance issues, cisterns that are belowground benefit from the cooler year-round groundtemperatures. To maximize efficiency, cisterns should

be located as close to both the supply and demandpoints as possible. And, to facilitate the use of gravityor lower stress on a pump, the cistern should be placedon the highest level that is workable.

While the catchment area (roof) should not beshaded by trees, the cistern can benefit from the shadesince direct sunlight can heat the stored rainwater inthe tank and thereby encourage algae and bacterialgrowth, which can lower water quality.

Texas does not have specific regulations concerning

rainwater systems; however, to ensure a safe watersupply, cisterns should be sited at least 50 feet awayfrom sources of pollution such as animal stables, latrines,or, if the tank is below ground, from septic fields.

Tank placement should also take into considerationthe possible need to add water to the tank from anauxiliary source, such as a water truck, in the eventyour water supply is depleted due to over-use ordrought conditions. For this reason, the cistern shouldbe located in a site accessible to a water truck, prefer-ably near a driveway or roadway, and positioned toavoid crossing over water or sewer lines, lawns orgardens.

DESIGN FEATURES

Regardless of the type of tank material you select, thecistern should have a du rable, watertight exterior and aclean, smooth interior, sealed with a non-toxic joint

sealant. If the water is intended for potable use, thetank should be labeled as FDA-approved (Food and

Drug Administration), as should any sealants or paintsused inside the tank. A tight-fitting cover is essential toprevent evaporation, mosquito breeding, and to keepinsects, birds, lizards, frogs and rodents from enteringthe tank. If the cistern is your only water source, aninflow pipe for an alternate water source is advisable.All tanks, and especially tanks intended for potableuse, should not allow sunlight to penetrate or algaewill grow in the cistern. A settling compartment,which encourages any roof run-off sediment that mayenter the tank to settle rather than be suspended in thetank, is an option that can be designed into the bottomof the cistern.

Designing a system with two tanks provides someflexibility that may be of value. In most cases, anadditional tank represents added cost, regardless of whether it represents increased capacity. This is be-cause two smaller tanks of, for examp le, 1,500 gallonseach are generally more expensive than a single 3,000

gallon tank. The primary benefit of a multi-tank system isthat the system can remain operational if one tank has tobe shut down due to maintenance or leaking.

Regardless of tank type chosen, regular inspectionand proper maintenance are imperative to ensurereliability and safe, efficient operation. Remember thatwater is heavy. A 500 gallon tank of water will weighmore than two tons, so a proper foundation andsupport are essential.


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MATERIALS

Tanks are available in a range of materials andsizes, new and used, large and small, to accommo-date your system design and budget. For smallinstallations, readily available new and used tanks,including whiskey barrels, 55-gallon drums, andhorse troughs can be fashioned into supplementaldo-it-yourself systems. If used tanks are selected,be sure that they did not contain any toxic sub-stances which could affect water quality for many,many years. For large installations, many optionsexist for manufactured and site-built systems, asdescribed below.

Concrete and Masonry

Concrete. Reinforced concrete tanks can be builtabove or below ground by a commercial contrac-tor or owner-builder. Because of their weight, theyare usually poured in place to specifications andare not portable. How ever, concrete tanks can alsobe fashioned from prefabricated components,such as septic tanks and storm d rain culverts, andfrom concrete blocks. Concrete is durable andlong-lasting, but is subject to cracking; below-ground tanks should be checked periodically for

leaks, especially in clay soils where expan sion andcontraction may p lace extra stress on the tank. Anadvantage of concrete cistern chambers is theirability to decrease the corrosiveness of rainwaterby allowing the dissolution of calcium carbonatefrom th e walls and floors.

Ferrocement. Ferrocement is a term used to de-scribe a relatively low-cost steel-mortar compositematerial. Its use over the past 100 years has beenmost prevalent in developing countries in a rangeof low-cost applications, such as water tanks. It hasalso gained popularity among do-it-yourselfers inTexas and throughout the U.S. Although it is aform of reinforced concrete, its distinctive charac-teristics relative to performance, strength, and flex-ible design potentials generally warrant classifica-tion of ferrocement as a separate material. Unlike

reinforced concrete, ferrocements reinforcement iscomprised of multiple layers of steel mesh (oftenchicken w ire), shaped around a light framework of rebar, that are impregnated with cement mortar.Because its walls can be as thin as 1, it uses lessmaterials than conventional poured-in-place con-crete tanks, and thus can be less expensive.Ferrocement lends itself to low-cost constructionprojects, since it can take advantage of self-helplabor and p revalent, low-cost raw materials such asrebar, chicken wire, cement and sand. Ferrocementtanks are likely to require greater ongoing mainte-nance than tanks constructed of other materials.Small cracks and leaks can be easily repaired with amixture of cement and water, and also applied

where wet spots appear on the tanks exterior. Somesources recommend that it is advantageous to paintabove-ground tanks white to reflect the suns rays,reduce evaporation, and keep the water cool.Though ferrocement is most commonly a site-builtmethod, commercially available ferrocement tanksare available in some parts of Texas. Check to besure that the ferrocement mix does not contain anytoxic compounds which m ay make the w ater unfitfor use.

Stone. Across the Texas Hill Country and other partsof the state with abundant rock, site-built stonecisterns were historically a logical approach to tankfabrication since the materials were locally avail-able. The mass of the stone walls helps to keepinterior water temperature cool, and the tanks canbe designed to blend in with adjacent buildings.Some recent installations, such as the NationalWildflower Research Center in Au stin, have contin-ued the tradition of stone cisterns. As with cementtanks, these installations are permanent. Construc-tion procedures should be careful to exclude anycompounds which may be toxic, such as some typesof mortars and sealants, especially if the system isplanned for potable water.


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12


their relatively low cost and long life expectancythey are considered slightly more durable than fiber-glass with comparable life expectancy. Their lightweight makes them easy to transport and relocate, if needed, while their smooth interior surface makesthem easy to clean. Repairs are relatively easy tocarry-outuse heat to soften the plastic and reshapeas necessary. To ensure their long-life, polyethylenetanks shou ld be chosen which have ultra-violet (UV)inhibitors for outdoor use, or can be placed in anenclosure or painted with a protective surface toprovide protection from the sun. Black tanks havethe greatest UV resistance, with a life expectancy of 25 years, though will tend to absorb heat and thuscan affect water quality. Painting or shading the

tank will minimize the effects of UV light and isrecommended. Again, light penetration will pro-mote algae grow th. If you intend to use the tank forpotable water, be sure that it is FDA approved.

MetalGalvanized Steel. Steel tanks were a predominatechoice by those early Texans who did not have stonenearby, and continue to be a popu lar choice in Texastoday. Galvanized steel tanks are commercially

available and reasonably priced. They are noted fortheir strength, yet are relatively lightweight and easyto move. Corrosion can be a problem if exposed toacidic conditions; some suppliers provide an insideliner to guard against this problem. In addition, highand low pH water conditions can result in therelease of zinc. As with other tank materials, be surethat any galvanized metal tank used as a potablewater source is FDA approved. If salvaging an oldmetal tank, be aware that these were generallysoldered with lead and should not be used as apotable water source.

Wood Redwood and Cypress.Redwood is considered one of the most du rable woods for outdoor use, though isuncommon in Texas since it is not a native woodspecies. Cypress is a native Texas wood with many

of the same properties as redwood. Although cy-press was used to construct cisterns in Texas in theearly 1900s, cypress tanks are not commerciallyavailable today. Redwood has a reputation asdurable water storage tank material, and is attrac-tive because it has no resins that could affect theodor or taste of water, has high levels of tannin, anatural preservative which makes the tank resistantto insects and decay, and has a cellular constructionwhich allows for complete saturat ion from capillaryand direct pressure and enhances its capacity toretain moisture. In addition, redwood is an efficientinsulator, which keeps water cooler in summer andprotects it from freezing temperatures in winter,does not rust or corrode and requires no painting or

preserving. Redwood tanks have an average lifeexpectancy of 50 years, with some known to last aslong as 75 years.

D. CONVEYINGRemem ber, water only flows dow nhill unless youpump it. The old adage that gravity flow worksonly if the tank is higher than the kitchen sinkaccurately portrays the physics at work. The waterpressure for a gravity system d epends on the differ-

ence in elevation between the storage tank and thefaucet. Water gains one pound per square inch of pressure for every 2.31 feet of rise or lift. Manyplum bing fixtures and app liances require 20 psi forproper operation, while standard municipal watersupply p ressures are typically in the 40 psi to 60 psirange. To achieve comparable pressure, a cisternwould have to be 92.4 feet (2.31 feet X 40 psi = 92.4feet) above the homes highest plumbing fixture.That explains why pumps are frequently used,much in the way they are used to extract well water.Pumps p refer to pu sh water, not pu ll it.

To approximate the water pressure one would getfrom a municipal system, pressure tanks are ofteninstalled with the pump. Pressure tanks have a pres-sure switch with ad justable settings between 5 and 65psi. For example, to keep your in-house pressure atabout 35 psi, set the switch to turn off the pump when


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TREATMENT TECHNIQUES

METHOD LOCATION RESULT

SCREENING

Strainers and Leaf Screens Gutters and Leaders Prevent leaves and other debrisfrom entering tank

SETTLING

Sedimentation Within Tank Settles particulate matter

FILTERING

In-Line/ Multi Cartridge After Pump Sieves sediment

Activated Charcoal At Tap Removes chlorine*

Reverse Osmosis At Tap Removes contaminants

Mixed Media Separate Tank Traps particulate matter

Slow Sand Separate Tank Traps particulate matter

DISINFECTING

Boiling/ Distilling Before use Kills microorganisms

Chemical Treatments

(Chlorine or Iodine) Within Tank or At Pump Kills microorganisms(liquid, tablet or granule)

Ultraviolet Light Ultraviolet light systems Kills microorganismsshou ld be located after theactivated carbon filter before trap

Ozonation Before Tap Kills microorganisms*Should only be used after chlorine or iodine has been used as a disinfectant. Ultrav iolet light and ozone systems should be located after the activated carbon filter bu t befor e the t

the pressure reaches 40 psi and turn it on again whenthe pressure drops down to 30 psi.

E. WATER TREATMENT Before making a decision about what type of water treatment method to use, have your water tested by an

approved laboratory and determine whether your water will be used for potable or non-potable uses.

The types of treatment discussed are filtration,disinfection, and buffering for pH control. Dirt,rust, scale, silt and other suspended particles, birdand rodent feces, airborne bacteria and cysts willinadvertently find their way into the cistern orstorage tank even when design features such as roof washers, screens and tight-fitting lids are properlyinstalled. Water can be unsatisfactory withoutbeing unsafe; therefore, filtration and some form of disinfection is the minimum recommended treat-

ment if the water is to be used for hum an consump-tion (drinking, brushing teeth, or cooking). Thetypes of treatment units most commonly used byrainwater systems are filters that removesediment, in consort with either an ultraviolet lightor chemical disinfection.

FILTERSFiltration can be as simple as the use of cartridgefilters or those used for swimming pools and hottubs. In all cases, proper filter operation and mainte-nance in accordance with the instruction manual forthat specific filter must be followed to ensure safety.

Once large debris is removed by screens androofwashers, other filters are available which help

improve rainwater quality. Keep in mind that mostfilters on the market are designed to treat municipal


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How Does a Rainwater Harvesting Sy stem Work?

water or well water. Therefore, filter selection re-quires careful consideration.

Screening, sedimentation, and prefiltering occurbetween catchment and storage or within the tank. Acartridge sediment filter, which traps and removesparticles of five microns or larger is the most com-mon filter used for rainwater harvesting. Sedimentfilters used in series, referred to as multi-cartridge orin-line filters, sieve the particles from increasing todecreasing size.

These sediment filters are often used as a pre-filter for other treatment techniques such as ultra-violet light or reverse osmosis filters which canbecome clogged from large particles.

Unless you are adding something to your rain-

water, there is no need to filter out som ething that isnot p resent. When a d isinfectant such as chlorine isadded to rainwater, an activated carbon filter at thetap m ay be used to remove the chlorine prior to use.Remember tha t activated carbon filters are subjectto becoming sites of bacterial growth. Chem icaldisinfectants such as chlorine or iodine must beadded to the water prior to the activated carbonfilter. If ultraviolet light or ozone is used fordisinfection, the system should be p lacedafter the

activated carbon filter. Many water treatmentstandards require some typ e of disinfection afterfiltration with activated carbon. Ultraviolet lightd isinfection is often the meth od of choice. Allfilters must be replaced per recommended schedulerather than when they cease to work; failure to doso may result in the filter contributing to the water scontamination.

DISINFECTIONUltraviolet Light (UV) water disinfection, a physicalprocess, kills most microbiological organisms thatpass through them. Since particulates offer a hidingplace for bacteria and microorganisms, p refilteringis necessary for UV systems. To determine whetherthe minimum dosage is distributed throughout thedisinfection chamber, UV water treatment unitsshould be equipped with a light sensor. Either an

alarm or shu t-off switch is activated w hen the waterdoes not receive the adequate level of UV radiation.The UV unit m ust be correctly calibrated and testedafter installation to insu re that the water is beingdisinfected. Featured in the case stud ies are severalsystems which utilize ultraviolet light.Ozone is the disinfectant of choice in manyEuropean countries, but it has not been used inAmerican water treatment facilities until recently.Ozone is a form of oxygen (03) produced by passingair through a strong electric field. Ozone readilykills microorganisms and oxidizes organic matter inthe water into carbon dioxide and water. Anyremaining ozone reverts back to d issolved oxygen(02) in the water. Recent developments have

produced compact ozone units for home use. Sinceozone is produced by equipment at the point of usewith electricity as the only input, many rainwatercatchment systems owners use it to avoid having tohandle chlorine or other chemicals. Ozone can alsobe used to keep the water in cisterns "fresh". Whenused as the final disinfectant, it should be addedprior to the tap , but after an activated carbon filter,if such a filter is used .Chlorine or iodine for disinfecting. Private systems do

not d isinfect to the extent of public water systemswhere the threat of a pathogenic organism such ase. coli can affect many households. If the harvestedrainwater is used to wash clothes, water plants, orother tasks that do not involve direct hum anconsumption or contact, treatment beyondscreening and sedimentation removal is optional.How ever, if the water is plumbed into the house forgeneral indoor use such as for drinking, bathing,and cooking, disinfection is needed.

While filtering is quite common in p rivate watersystems, disinfection is less common for thesereasons: the Safe Drinking Water Act is neitherenforced nor app licable to private systems; chlorineis disliked du e to taste, fear associated withtrihalomethanes (THMs), and other concerns. Chlo-rine is the most common disinfectant because of itsdependability, water solubility, and availability.


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Granular or tablet form is available (calciumhypochlorite), but the recomm ended application forrainwater disinfecting is in a liquid solution(sodium hypochlorite).

Household bleach contains a 5.0% solution of sodium hypochlorite, and is proven to be reliable,inexpensive and easily obtained. A dose is oneliquid ounce of bleach for each 100 gallons (one anda quarter cups of bleach per 1,000 gallons) of rain-water collected will most likely be sufficient todisinfect the collected rainwater. When disinfecting,never overdose with bleach. Mixing occurs natu-rally over a day or so, but a clean paddle may beused to accelerate the p rocess.

When chlorine bleach is added directly to the

storage tank or cistern as described above, thechlorine will have a longer time to kill bacteria thusachieving a better rate of disinfection. Chlorine feedpumps which release small amounts of solutionwhile the water is being pumped can also be used.Chlorine metering pumps inject chlorine into thewater only at the time of use.

Chlorine concentrations are easily measured witha swimming pool test kit. A level of between 0.2mg/ L (milligrams per liter) and 1.5 mg/ L is recom-

mended . If the level is below 0.2 mg/ L, add oneliquid ounce of chlorine bleach per 100 gallons of thevolume of water in storage (one and a quarter cupsper 1,000 gallons) if you are using bleach or adjustthe chemical feed pump in accordance with thepumps instructions.

Swimming pool test kit chemicals are toxicand should never be allowed to mix with cistern water.Testing should occur outside the tank.

Chlorine is more effective at higher water tem-peratu res and lower pH levels than iodine. Iodine isanother water disinfectant that is less soluble thanchlorine although it is effective over a p H range of 5to 9 and displays greater antibacterial activity inwater temperatures of 75 to 98.6 degrees Fahrenheit.

Prolonged presence of chlorine where organicmatter may be present may cause the formation of chlo-rinated organic compounds. If chlorine is used as a dis-infectant, be sure to screen all organic material from thetank.

BUFFERING Baking soda for buffering. The composition and pH of rainwater differs from chemically treated municipalwater and mineral rich well water. Contro lling thepH of rainw ater by buffering can be easily accom-plished by adding one level tablespoon of bakingsoda to the storage tank for each 100 gallons of watercollected. (About four ounces by weight of bakingsoda for every 1,000 gallons of water collected.) Aneasy method is to mix this amount of baking soda in

a jar of water and pour it into the tank. Mixing willoccur naturally over a day or two or a clean paddlemay be used to hasten the process, but avoid d isturb-ing materials that have settled at the bottom of thecistern.

OTHER TREATMENTThere are a number of other treatment devices

available on the market. When selecting add itionaltreatment devices, always ask yourself what is itthat you are trying to remove, does it need to beremoved, and does this water source contain thatcontaminant. Comm ercial and public test laborato-ries can help in this regard.

Some of the types of treatment available includereverse osmosis (RO) and nano-filtration, and sev-eral other membrane processes and distillationequipment that are designed primarily to removedissolved materials such as salts or metals, but

rainwater contains extremely low d issolved salts orhardness levels. For the most part, systems such asRO would be redund ant and expensive to use.Besides, most home RO units waste three to fivegallons of water for every gallon of water produced.

As a word to the wise, consult your local healthdepartment before pu rchasing such devices. Somedevices are actually dangerous if used incorrectly.


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ssessing your indoor and outdoor w aterneeds will help determine the best usefor the rainwater. If you are alreadyconnected to a municipal water system,

then a rainwater harvesting unit designed to fulfilloutdoor requirements such as lawn and gardenirrigation may be most cost-effective. If you have

already invested in a well-water system, rainwatercould augment or enhance the quality of mineral-ized well water for purposes such as washing, orprovide back-up water when underground watersources are low. Some people are installing a full-service rainwater system designed to supply boththeir indoor and outdoor water needs. If you are

*All of the flow rates shown are for new fixtures. Older toilets use from 3.5 to 7 gallons p er flush, and older show er heads have flowrates as high as 10 gallons per minute.

HOUSEHOLD WATER DEM AND CHART

F I X TURE USE FLOW RATE # OF USERS TOTAL

Toilet # flushes per person per day 1.6 gallons per flush(new toilet)*

Shower # minutes per person per day 2.75 gallon per minute*(5 minutes suggested max.) (restricted flow head)

Bath # baths per person per day 50 gallons per bath(average)

Faucets bathroom and kitchen sinks 10 gallons per day not applicable(excluding cleaning)

Washing # loads per day 50 gallons per load not applicableMachine (average)

Dishwasher # loads per day 9.5 gallons per load not applicable

Total # gallons/ day

multip ly (x) 365 # gallons/ year

V. HOW MUCH WATER DO YOU USE?

Aconsidering this option, it is imperative that youemploy best conservation practices to ensure a year-round water supply. Three variables determineyour ability to fulfill your household water de-mand: your local precipitation, available catchmentarea, and your financial budget.

If you are accustomed to simply turning on a tap

to get your water and then paying a bill at the endof the month, the switch to a rainwater system willrequire some adjustment. While the associatedtasks are not difficult, they are important to keepyour water safe and your family in good health.These responsibilities include regular inspections of all the previously discussed components, including



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pruning branches that overhang roof, keeping leaf screens clean, checking tank and pump, replacingfilters, and testing the water. A maintenanceschedule and checklist based upon your partic-ular system are recommended to ensure properperformance.

HOUSEHOLD WATER BUDGETAn easy way to calculate your daily water consump-tion is to review previous water bills, if you pres-ently receive municipal water. Another m ethod is toaccount for every water-using activity, includingshower, bath, toilet flush, dishwashing run, washingmachine load. A conserving household that has low-flow plumbing fixtures such as 1.6 gallon-per-flushtoilets and 2.75 gallon-per-minute shower heads,now required by the Texas Plumbing Standards,might use 55 gallons or less of water per day perperson and very conservative minded householdsmight be able to reduce water u se to as low as 35gallons p er person p er d ay. However, for the pur-poses of designing a rainwater system, an estimateof 75 gallons per person per day for indoor use isadvised to ensure adequate year-round indoor wa-ter supply unless you are sure that all of your

fixtures are the newer, more efficient ones and youplan to follow strict conservation practices. Com-plete the Household Water Consumption Chart onpage 16 to see how your households water con-sump tion compares with the recommended designallowance. See page 18 for outdoor use estimates.

While inside water use remains relatively levelthroughout the year, total water demand increasesduring the hot, dry summ ers due to increased lawnand garden watering, and decreases during the cool,wet winters when the garden is fallow and the lawnneeds little attention. To determine your daily waterbudget, multiply the number of persons in thehousehold times the average water consumption.Estimates of indoor household water use rangefrom less than 55 gallons per person a day in a

Peak

Winter Summer Winter

DailyWaterUse

SeasonalVolume 25%

Base Volume 75%

65%

35%

BASE AND SEASONAL WATER USE IN TEXAS

Toilet12%

Bathing29%

Bathing30%

Leaks8%

Leaks5%

Dishes4%Dishes

3%

Laundry31%

Laundry22%

Toilet28%

Faucets16%

Faucets12%

Homes with water-saving fixtures use about55 gallons per person p er day (GPCD)

Hom es with older fixtures use abou t 75gallons per person per day (GPCD)

HOME I NDOOR WATER USE

How M uch Water Do You Use?


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HOME WATER USE, INDOOR AND OUTDOOR

conservation minded household to well over 75gallons per person a day in non -conserving house-holds.

LANDSCAPE WATER BUDGETIn order to calculate a water budget for a conven-tional lawn, you must determine the grass type, thesquare footage of your lawn, and your annualrainfall. If the average annual rainfall for your area is

higher than the required water demands listed be-low, your annu al rainfall is sufficient. If your annualrainfall is lower than the required inches based ongrass type, you w ill need to comp lete the followingchart to determine your lawn watering require-ments in order to properly size your cistern.

WATER CONSERVATION TECHNIQUESWhile rainwater collection can function well as a stand alone system, its efficiency can be enhanced by workingin concert with other water conservation practices.

Reducing your water demand results in lowering theup front cost of your rainwater harvesting system.

SAVING WATER INSIDE YOUR HOUSEIf your water budget or water bill indicates usagebeyond your collection capacity, comm on sense w a-ter conservation practices might help you to recoverthose extra gallons. The repair of dripping faucetsand leaking toilets, frequently the source of muchlost water, is a good start. Installing low-flowshowerheads, faucet aerators and toilet dams areother steps that pay for themselves in less than ayear through water savings. Water conserving dishwashers and clothes washing machines that operate

with half as much water as conventional appliances

Rest of Bathroom23%

Toilet26%

Cleaning2%Lawn

35%

Laundry9%

Kitchen5%

GRASS TYPE AND THEIR WATER DEMAND

St. Augustine 50 inches per year Bermuda 40 inches per year St. Augustine/ 45 inches per yearBermuda Mix

Buffalo Grass 25 inches per year Zoysia 45 inches per year

1. Multiply the w ater demand (inches per year) times your lawn size (square feet) and divide by 12. Thiswill give you the cubic feet of of water demand per year. cu. ft.

2. Multiply the num ber cubic feet of water demand per year (line 1) times a conversion factor of 7.48.Th is gives you the number of requ ired gallons of water per year. gal.

3. Multiply the inches of natural rainfall for your area (see page 20) times your lawn size (square feet)and divide by 12. This gives you the cubic feet of water supplied by natural rainfall. cu. ft.

4. Multiply the cubic feet of natural rainfall times a conversion factor of 7.48. This gives you the gallonsof natural rainfall per year. gal.

5. Subtract the gallons of natural rainfall (line 4) from the required water demand for your grass typ e(line 2). This gives you the gallons required. gal.

LANDSCAPE WATER DEMAND CHART



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How M uch Water Do You Use?

AVERAGE ANNUAL PRECIPITATION IN TEXASIn inches, 1961-1990

8

8

10

12

14

16

16 18 20

2426

28 32 36 4044 46

48

56

5652

48

4848

44 4640

36

32

28

262422

2018

52

52

22

22

22

14

14

10

10

1216

1614

18

10 1214 16

are also available. The Texas Water DevelopmentBoard and your local water utility have more infor-mation on ways to conserve water in your bath-room, kitchen, and laund ry.

SAVING WATER OUTSIDE YOUR HOUSELandscape irrigation accounts for about one-quarterof all mun icipal water use in Texas. The most inten-sive time to irrigate is the summ er grow ing season,which is when temperatures are highest and rainfallis lowest. Rainwater becomes particularly preciousduring these hot, dry months. Indeed, rainwater

used for summer irrigation must be captured earlierin the year. Therefore, a landscape that requiresminimum watering, especially in the summer, ismost appropriate for rainwater harvested irrigation.The use of regionally-adapted drought tolerant andlow water use plants is also a major help .

Drip Irrigation. Trickle or drip irrigation is thefrequent, low pressure application of smallamou nts of water to the soil area directly surround-ing the plant roots. A constant level of soil moistureis maintained, even though up to 60% less water


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7 WATER WISE PRINCIPLES

1. Planning & Design that considers topog raph y, existing vegetation, and group ing plantsand grasses by their watering needs.

2. Soil Improvem ent to preven t erosion and ad ding o rganic material, such ascompost, to promote water penetration and retention.

3. App ropriate Plant Selection such as native and adap ted p lants that u se lesswater and are more resistant to diseases and pests.

4. Practical irrigated tu rf and landscaped areas in app ropriate locations to be

separately irrigated.

5. Efficient Watering b y avoid ing w atering u ntil absolutely necessary andnever watering in the heat of the day or on windy days to avoid evaporation.

6. Use of Mulches to cover and shad e soil, minimize evapor ation, reduceweed growth and soil erosion.

7. Lower Maintenance by the d ecreased u se of pesticides and fertilizers.


than conventional watering is used by this method .The efficiency and uniformity of a low water flowrate reduces evaporation, run-off, and deep perco-lation. A common soaker hose, usually installedbelow ground, is one of the simplest ways to dripirrigate shrub beds, gardens and young trees.

To obtain more information about Water Wiselandscaping, drip irrigation, and indoor water conserva-tion techniques, contact the Texas Water Development

Board at Conservation, P.O. Box 13231, Austin, TX 78711-3231 or the Texas Department of Agriculture,city utility, river authority, or your county agricultureextension agent.

Greywater Reuse. In urban areas, public policy and

health codes generally mandate the centralized col-lection and treatment of household wastewater.Policy discussions relating to greywater reuse areunderway, reflecting concern to maximize water useoptions brought on by droughts, water shortages,and development impacts on existing wastewatertreatment facilities. Greywater reuse, which relies onseparating the greywater from the blackwater, hasmany environmental and economic benefits on boththe building and regional scales. Because greywateris relatively benign, it can be directed to a number of secondary uses such as toilet flushing and irrigation,

thus displacing the need to use higher quality water.Greywater is household wastewater generated by

clothes washing machines, showers, bathtubs, andbathroom sinks. Wastewater from kitchen sinks isexcluded from this category since it contains oil, fat,and grease which are difficult to filter, clog distribu-tion pipes, have unpleasant odors, and are likely toattract pests.

Blackwater is the water flushed down toilets andurinals and also includes the discharge from kitchensinks due to the reasons stated above. If a sanitarysewer connection is not available, blackwater mustbe treated on site by a septic tank, drain field, or apermitted on-site wastewater treatment system.

Greywater can contain harmful bacteria and

therefore also requires filtration and disinfectionprior to reuse. Once the greywater is properlytreated, it can be reused for irrigation and used tosupp lement h igher quality rainwater. Always con-sult your local health department.

If you plan to incorporate a greywater system, check with your local health department officialssince certain restrictions regarding installation and reuseapply. For example, greywater systems with overflows to

public sewer systems cannot be connected to rainwater systems.


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How Much Water Do You Use?

Other Water Reuse OptionsUnder new rules from the Texas Natural Re-

sources Conservation Commission (TNRCC), wa-ter from on-site sewage facilities, such as septicsystems, can n ow be reused for land scape irriga-tion after proper secondary treatment and disin-fection. Contact your local health department orthe TNRCC at P.O. Box 13087, Austin, Texas78711-3087. The applicable rule is 30 TAC285. Youcan also download the rule from the TNRCC's website at http:/ / ww w.tnrcc.state.tx.us.


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ow that you have a betterunderstanding of the principles of rainwater catchment, the next questionsare, how much rain can you expect to

collect in your location and how reliable is thisrainfall. The simple answer to the first question isthat one inch of precipitation (1/ 12 foot) on onesquare foot of collection area equals 0.6233 gallons.Many simply round this off to 600 gallons collectedper inch of rain on 1,000 square feet. From thisbasic rule of thumb (600 gallons per inch on 1,000square feet) the analysis shifts to (1) how efficientlycan this rainfall be collected, and (2) how reliable isthe rainfall on your specific area. Once thesequestions are answered, you will need to balancethe amou nt of rainfall than can be collected withthe amou nt of water that w ill be used. You may

be surprised to learn that even with the strictestwater conservation measures, rainfall collectioncan only provide a fraction of the amount of wateryou use.

The answer to these questions also depends inpart on what the harvested rain will be used for. If it is to provide supp lemental water for the yard, theanswer will be different than if the system will bethe sole source of water for a household. Oneshould also keep in mind that the efficiency of thecollection system can change depending on designwhile the question regarding precipitationreliability depends on where you are located.Collection Efficiency. How efficiently the rainfall canbe collected depends on several considerations.Many first assume that I can collect all of it, butthis is never the case. First, there is always a small

loss to rainfall needed to wet the roof area andwater collected by the roof washer. This is usuallya small percentage of the rainfall and will rangefrom about 3/ 100s to 1/ 10th of an inch per rainfallevent, depending on the roof material and thevolume the roof washer diverts. Built-up flat roofscan retain as much as half an inch of water

depend ing on their condition and design. Overshotof gutters and spillage during very intensiverainfall events w ill occur.

Spills and rate of rainfall can also make adifference. If filter type roof washers are used, theywill spill the excess flow once the filter flowthrough capacity is exceeded. Finally, you cancollect only as much rainfall as your storagesystem will hold. Depending on your design,most cisterns will become full during especiallyrainy periods and any additional rainfallcollected will spill. Collection efficiencies of 75%to 90% are often used by installers dep ending onthe specific design if the system is to providewater for in-home use. For small systemsdesigned for supplemental plant watering,collection factors of below 50% are commonbecause it is not economic to install the largestorage that would be required to increase this

factor. Rainfall Reliability. The reliability of precipi-tation requ ires a closer look. The first and sim-plest param eter to consider is average precipi-tation. The map on page 19 shows averageprecipitation for Texas. The first step oneshould take is to use the average rainfall foryour area to determine how much water

VI. HOW MUCH RAINFALL CAN YOU COLLECT?

N



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How Much Rainfall Can You Collect?

would be generated from you r roof area. Thecalculation is the roof catchment area timesthe average rainfall times 600 gallons d ividedby 1,000.

Area (ft 2) X Average Rainfall (inches) X 6001,000

If you are only interested in supp lemen-tal water for plant w atering, this may be su f-ficient knowledge, but if rainwater is to beyour sole source of wa ter, you need to knowwhat p recipitation rate you can rely on in moredetail. Once your system is in, you w ill needto know the amount of rainfall you can ex-

pect from one m onth to the next.The figure on p age 24 shows annu al pre-

cipitation for seven cities from across Texas.The annual rainfall is arranged in rank ord erfrom the lowest to the highest for each city.This graph also shows the p ercent of time thatrainfall of that magnitude will not be ex-ceeded . For example, the graph shows that inWichita Falls, rainfall will be greater than 30inches per year about 35% of the time or lowerthan 30 inches per year 65% of the time. The

graph also indicates that annual rainfall inWichita Falls w ill fall between 17 inches bu tbelow 37 inches 90% of the time.

As a rule-of-thumb in Texas, if one di-vides average ann ual rainfall by tw o, the re-sulting answ er w ill be near the 5% percentilerainfall. For example, Austin receives an av-erage of 32 inches a year and only 5% of thetime is rainfall less than 17 inches a year. Thiscan help give a quick method of determ iningif enough rainfall will occur to p rovide w aterdu ring very low periods based on ann ual pre-cipitation. An expample of how annual datacan be used is shown below.

The monthly distribution of rainfall is

also important information for sizing a sys-tem. Using ann ual data d oes not tell you howmuch water one can expect from one monthto the next or just as important, once in op-eration, how mu ch rainfall can one expect inany one given m onth. These statistics are p re-sented in Rainfall Data for Selected Com mu -nities Across Texas on page 27.

The use of these data is twofold. First, the10%, 25% and 50% (median) monthly data

present the p ercent of times that rainfall is less

BASIC METHOD USING ANNUAL DATA

1. Calculate Roof Catchment Area (see page 7)

2. Multiply the collection area in square feet by 0.6 gallons per squ are foot per inch of raintimes the collection factor times the average annual rainfall and half of the average an-nual rainfall.

For example, if you have 2,500 square feet of collection area and live in A ustin , where the

average annual rainfall is 32 inches a year and the collection efficiency factor is 80%, the aver-age amount of rain you can collect is:

2,500 X 0.6 X 0.8 X 32 = 38,400 gallons per year

3. Dividing this by 365 days a year, the sup ply w ould be 105 gallons per d ay.

4. Using the ru le-of-thu mb that half of the average rainfall will provide a close estimate of the low expected rainfall for the area, in an extremely severe drought year, app roxi-mately 19,700 gallons could be collected. This wou ld resu lt in a supp ly of only 53 gallonsa day.


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MON THLY BALAN CE CALCULATION S

than that value. Examination of the data alsoshows that the 50% values are lower than theaverage values. This is because the arithm eticaverage is skewed by a few abnorm ally highrainfall events such as those occurring dur-ing hurricane. While the median representsthe rain fall tha t, when all historic rainfall val-ues for that month are ranked from lowest tohighest, is in the mid dle.

The way to use these data on a mon thlybasis is that for any given mon th, you can ex-pect to get at least the med ian rainfall half of the t ime, the 25% rainfall 75% of the t ime, andthe 10% rainfall 90% of the time. The sum of the twelve monthly median values is lower

than the annual average as explained above.In Texas, the sum of the medial values pro-vides a rainfall that can be relied on for 65%of the time or more. For example, the total forAustin is 25 inches which occurs 80% of thetime in any given year an d the 25% monthly

value annual total is 12 inches which occursover 95% of the t ime.

This information can also be used to d e-velop month ly balances of dem and and stor-age as shown on the following page usingAustin d ata. In th is examp le, the median (50%)and 25% monthly rainfall were used . The pur-pose is to determine how much storage ca-pacity is needed an d the level of dem and thatcan be sustained .

Different storage volumes, roof sizes (if this is an option), and monthly demands aretried . In the exam ple case, the roof size is 3,000square feet, the collection efficiency is 80%,and the storage volum e is 10,000 gallons. It is

assumed that the year begins with 3,000 instorage; monthly demands of 2,000 gallons,3,000 gallons, and 4,000 gallons are used . Thecalculation is done by following the steps be-low.

1. Determine Janu ary rainfall for both the 50% and 25% levels. For example, at the 50% rain-

fall level of 1.23 inches for January it is:3,000 X 0.8 X 1.23 X 0.623 = 1,839 gallons collected

2. Add the volume already in storage (3,000 gallons) to the gallons collected and subtract themon thly demand . For the 2,000 gallons a m onth dem and example and 50% rainfall level,this is:

1,839 + 3,000 - 2,000 = 2,839 gallons in storage at the end of the month

This is repeated, but n ote that if the storage is zero or less at the end of the month , use zerofor the next month; if the amount in storage at the end of the month is greater than th ecapacity of the cistern (10,000 gallons in this example) use th e storage capacity for th e end

of the month storage. The end result is that for the 10,000 gallon storage capacity an an vergeuse of 3,000 gallons/ mon th, but not 4,000 gallons/ mon th, could be sup ported .

Many professionals use 50 to 100 years of actual monthly rainfall data in a programwh ich p erforms the sam e series of calculations as d escribed above to determine the op ti-mum system size.



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Monthly Rainfall End of Mo. Rainfall End of Mo.Month Use 50% rain Collected St orage 25% rain Collected St or age

( gal/m o) ( inches) ( gallons) ( gallons) ( inches) ( gallons) ( gallons)

3,000 3,0001 2,000 1.23 1,839 2,839 0.60 897 1,8972 2,000 2.28 3,409 4,248 1.13 1,690 1,587

3 2,000 1.66 2,482 4,730 0.81 1,211 7984 2,000 2.18 3,260 5,990 1.38 2,063 8615 2,000 3.89 5,816 9,806 1.60 2,392 1,2546 2,000 2.63 3,932 10,000 1.51 2,258 1,5117 2,000 1.01 1,645 9,645 0.44 658 1698 2,000 1.19 1,779 9,424 0.60 897 09 2,000 3.15 4,710 10,000 1.50 2,243 24310 2,000 2.78 4,157 10,000 0.87 1,301 011 2,000 1.71 2,557 10,000 0.74 1,106 012 2,000 1.24 1,854 9,854 0.74 1,106 0

24,000 25.04 37,440 11.92 17,823

1 3,000 1.23 1,839 1,839 0.60 897 8972 3,000 2.28 3,409 2,248 1.13 1,690 03 3,000 1.66 2,482 1,730 0.81 1,211 04 3,000 2.18 3,260 1,990 1.38 2,063 05 3,000 3.89 5,816 4,806 1.60 2,392 06 3,000 2.63 3,932 5,738 1.51 2,258 07 3,000 1.01 1,645 4,383 0.44 658 08 3,000 1.19 1,779 3,162 0.60 897 0

9 3,000 3.15 4,710 4,872 1.50 2,243 010 3,000 2.78 4,157 6,029 0.87 1,301 011 3,000 1.71 2,557 5,586 0.74 1,106 012 3,000 1.24 1,854 4,440 0.74 1,106 0

36,000 25.04 37,440 11.92 17,823

1 4,000 1.23 1,839 839 0.60 897 02 4,000 2.28 3,409 248 1.13 1,690 03 4,000 1.66 2,482 0 0.81 1,211 04 4,000 2.18 3,260 0 1.38 2,063 05 4,000 3.89 5,816 1,816 1.60 2,392 06 4,000 2.63 3,932 1,749 1.51 2,258 07 4,000 1.01 1,645 0 0.44 658 08 4,000 1.19 1,779 0 0.60 897 09 4,000 3.15 4,710 710 1.50 2,243 010 4,000 2.78 4,157 867 0.87 1,301 011 4,000 1.71 2,557 0 0.74 1,106 012 4,000 1.24 1,854 0 0.74 1,106 0

48,000 25.04 37,440 11.92 17,823

EXAM PLE MONTHLY WATER BALANCE CALCULATIONS


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MONTH M IN. 10% 25% 50% AVE. MAX.

AbernathyJanuary 0.00 0.00 0.09 0.35 0.57 3.75February 0.00 0.01 0.12 0.45 0.67 2.28

March 0.00 0.01 0.17 0.44 0.72 3.13April 0.00 0.13 0.37 0.88 1.11 3.96May 0.31 0.68 0.98 1.81 2.47 6.32June 0.32 0.52 1.56 2.84 3.03 8.36July 0.00 0.46 1.20 2.38 2.44 9.68August 0.10 0.45 0.83 1.96 2.34 8.54September 0.05 0.34 0.74 1.81 2.27 6.43October 0.00 0.00 0.28 1.00 1.67 7.41November 0.00 0.00 0.05 0.37 0.61 2.08December 0.00 0.01 0.09 0.31 0.57 2.22

AbileneJanuary 0.00 0.00 0.10 0.79 0.98 4.29February 0.02 0.10 0.35 1.02 1.08 3.57March 0.02 0.13 0.41 0.79 1.08 5.08April 0.00 0.44 0.97 1.87 2.10 6.76May 0.14 0.70 1.44 2.98 3.36 13.11June 0.00 0.35 1.46 2.19 2.80 9.55July 0.00 0.23 0.86 1.71 2.16 7.11August 0.00 0.34 0.74 1.60 2.31 8.18September 0.00 0.50 1.23 2.31 2.79 10.97October 0.00 0.35 1.01 2.08 2.51 10.64November 0.00 0.00 0.33 0.76 1.23 4.55December 0.00 0.01 0.18 0.74 1.05 6.22

M ONTH M IN. 10% 25% 50% AVE. M AX

AckerlyJanuary 0.00 0.00 0.00 0.37 0.56 2.17February 0.00 0.00 0.09 0.44 0.63 3.20


AlbanyJanuary 0.00 0.00 0.11 0.87 1.27 8.06February 0.15 0.32 0.60 1.01 1.54 6.51March 0.06 0.15 0.51 0.99 1.42 4.31April 0.00 0.58 1.11 2.16 2.58 10.12May 0.24 1.22 2.20 3.58 3.94 10.46June 0.06 0.32 1.18 2.26 2.87 9.42July 0.00 0.12 0.66 1.79 2.26 11.52August 0.11 0.36 0.71 1.41 1.99 6.53September 0.00 0.22 1.62 2.69 3.35 13.40October 0.00 0.23 0.89 2.24 2.74 1.01November 0.00 0.00 0.36 0.81 1.43 6.07December 0.01 0.07 0.32 1.12 1.42 8.62

RAINFALL DATA FOR SELECTED COM M UNITIES ACROSS TEXAS

This chart contains monthly rainfall data from 40 weatherstations across Texas. The statistics are based on 50 years of recorded rainfall, from 1940 through 1990. For each station,six monthly precipitation values are given:s MIN. The minimum recorded occurrence is the lowest

recorded rainfall in 50 years.s 10% The 10% occurrence level indicates that 90% of thetime mon thly rainfall is higher.s 25% The 25% occurrence level indicates that 75% of thetime mon thly rainfall is higher.s 50% The 50% (median) occurrence level describesmon thly rainfall for ha lf the time.s AVE. The average monthly (mean) occurrence levelfactors in p recipitation extremes and is higher than the 50%(median) data.s MAX. The maximum recorded occurrence is the highestrecorded mon thly rainfall in 50 years.

Refer to the map below to see if there is a data set foryour town. If you live between weather stations, averagethe mon thly precipitation values for the closest town to th enorth and to the south , since precipitation pa tterns in Texasrun from the north to south/ southwest. (See map of Aver-age Annual Precipitation in Texas, page 19.)

Dalhart

Childress

AbernathyLubbock

AckerlyAlbany Dallas

De Kalb

Karnak

Longview

Lufkin

PortArthur

AnahuacHouston

WacoCameron

Austin

San Antonio

CollegeStation

GonzalesPalacios Galveston

Weatherford

Wichita Falls

Abilene

San Angelo

Brady

Hunt

Eagle Pass

Cotulla CorpusChristi

Falfurrias

Harlingen

Brownsville WSO

Midland/OdessaWink

AlpineBakersfield

El Paso

Amarillo



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Rainfall Data for Selected Communities Across Texas

MONTH MIN. 10% 25% 50% AVE. M AX.

AlpineJanuary 0.00 0.01 0.13 0.45 0.54 1.82February 0.00 0.00 0.03 0.24 0.47 3.04March 0.00 0.00 0.03 0.18 0.35 1.66April 0.00 0.01 0.07 0.25 0.51 3.60May 0.00 0.18 0.43 1.06 1.19 3.41June 0.00 0.41 0.89 1.76 2.04 6.93July 0.05 0.60 1.39 2.66 2.93 9.30August 0.05 0.84 1.52 2.27 2.64 8.15September 0.01 0.49 1.00 2.21 2.61 11.08October 0.00 0.11 0.28 0.98 1.28 4.39November 0.00 0.00 0.00 0.37 0.47 3.19December 0.00 0.00 0.09 0.28 0.52 2.56

AmarilloJanuary 0.00 0.00 0.11 0.42 0.53 2.30February 0.00 0.03 0.20 0.48 0.55 1.76March 0.00 0.02 0.26 0.59 0.88 3.93April 0.00 0.19 0.40 0.85 1.03 2.75May 0.01 0.74 1.40 2.47 2.69 9.76June 0.00 1.00 1.70 3.15 3.44 10.62July 0.11 0.79 1.46 2.50 2.77 7.50August 0.26 1.20 1.68 2.87 2.99 7.45September 0.02 0.31 0.68 1.59 1.84 4.95October 0.00 0.12 0.45 0.99 1.34 4.78November 0.00 0.00 0.14 0.39 0.60 2.23December 0.00 0.02 0.15 0.27 0.52 4.46

AnahuacJanuary 0.63 1.15 2.12 4.33 4.09 10.02February 0.00 0.90 1.77 2.81 3.36 10.93


AustinJanuary 0.02 0.35 0.60 1.23 1.79 9.14

February 0.23 0.59 1.13 2.28 2.40 6.48March 0.00 0.25 0.81 1.66 1.84 5.97April 0.03 0.53 1.38 2.18 2.89 9.85May 0.77 1.17 1.60 3.89 4.40 9.90June 0.00 0.66 1.51 2.63 3.41 14.87July 0.00 0.11 0.44 1.10 1.75 10.50August 0.00 0.25 0.60 1.19 2.03 8.84September 0.09 0.80 1.50 3.15 3.22 7.41October 0.00 0.56 0.87 2.78 3.50 12.25November 0.00 0.32 0.74 1.71 2.05 7.28December 0.00 0.32 0.74 1.24 2.18 14.05

M ONTH MIN. 10% 25% 50% AVE. MAX.

BakersfieldJanuary 0.00 0.00 0.02 0.31 0.61 4.24February 0.00 0.00 0.10 0.40 0.62 4.33March 0.00 0.00 0.02 0.22 0.42 1.83April 0.00 0.00 0.11 0.54 0.82 3.58May 0.00 0.43 0.89 1.30 1.76 4.56June 0.00 0.00 0.18 1.20 1.40 6.00July 0.00 0.03 0.14 0.81 1.21 6.23August 0.00 0.03 0.40 1.17 1.46 4.73September 0.05 0.22 0.60 1.64 2.49 23.41October 0.00 0.12 0.32 1.38 1.83 13.30November 0.00 0.00 0.00 0.35 0.57 2.61December 0.00 0.00 0.00 0.23 0.54 2.92

BradyJanuary 0.00 0.02 0.17 0.71 1.13 6.36February 0.09 0.34 0.70 1.33 1.57 5.19March 0.00 0.11 0.39 0.81 1.19 3.44April 0.26 0.52 1.08 1.88 2.13 6.45May 0.11 1.36 1.90 3.31 3.69 7.88June 0.00 0.42 0.81 1.89 2.60 8.24July 0.00 0.02 0.17 1.15 1.96 13.99August 0.07 0.28 0.63 1.34 2.08 11.12September 0.00 0.57 1.41 2.55 3.19 10.41October 0.01 0.18 0.64 1.78 2.47 7.68November 0.00 0.12 0.40 1.10 1.28 3.77December 0.00 0.05 0.14 0.76 1.29 8.16

BrownsvilleJanuary 0.00 0.13 0.36 1.07 1.37 4.74February 0.00 0.05 0.35 1.00 1.43 10.21

March 0.00 0.01 0.11 0.33 0.60 3.44April 0.00 0.00 0.23 0.88 1.65 10.33May 0.00 0.25 1.14 1.95 2.50 9.05June 0.00 0.06 1.34 2.20 2.80 8.45July 0.00 0.10 0.31 1.06 1.63 9.35August 0.00 0.12 0.88 2.10 2

rainwater harvesting 202

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